Following DNA damage or inhibition of replication, E. coli expresses many new cellular functions, including mutagenesis. Since many processes are expressed co-ordinately, biochemical analysis of any one process is difficult. Expression of these functions is controlled by a regulatory system which involves the products of at least two genes, recA and lexA. Understanding the functions of these proteins is a powerful approach to understanding other induced processes. The recA protein is known to have several functions, and these may account for some of the induced processes. Genetic studies have suggested that one important function of lexA protein is to regulate the expression of the recA gene, which is expressed at very high rates in cells with damaged DNA. The objective of the proposed research is to determine the biochemical functions of the lexA protein in the regulatory process. We have already identified this protein and shown that it is cleaved by a specific protease activity of the recA protein. We intend to purify the lexA protein and characterize its activity in vitro. In particular, we will test whether lexA protein is a repressor of recA, or if it plays a more indirect role in recA regulation. We will also study mutant proteins, and will characterize wild-type and mutant components of the cell with which lexA protein interacts. This combination of genetic and biochemical approaches should enable us to understand the biochemical role of lexA protein. We believe that this understanding is basic to further work in this field: it should help to dissect the mutagenesis pathway from other inducible processes; it may lead to discovery of similar regulatory systems in eukaryotic cells; it may provide the means for controlling induced mutagenesis; and it may uncover other cellular processes, also induced by mutagens and carcinogens, which will prove to play a significant role in carcinogenesis.